Problem: $\cos(-180^{\circ}) = \text{?}$
To find the cosine using the unit circle, first find the angle. Drag the orange point around the circle until $-180^{\circ}$ is selected. The correct angle is selected. Remember, the cosine of an angle is represented by the $x$ coordinate of a point on the unit circle. The $x$ coordinate of the point is $-1$, so $\cos(-180^{\circ}) = -1$.